1. Technical Field
The present disclosure relates to a receiver of a multiple input multiple output (MIMO) wireless communication system. More particularly, the present disclosure relates to a codebook searching apparatus of a receiver and a method thereof.
2. Description of Related Art
Present communication standards, such as IEE 802.16m and long term evolution (LTE) all support a closed loop multiple input multiple output (MIMO) wireless communication transmission technique, and an important technique thereof is to implement a pre-coding technique through a codebook. Theoretically, the more the pre-coding members (i.e. pre-coding matrices) are, the more complicated the required operation is. Presently, a plurality of rules can be used to determine whether a pre-coding matrix is suitable.
There are three documents related the codebook searching methods, one is “Proposal for IEEE 802.16m Differential Encoding/Decoding for CL-MIMO Codebook Feedback” published in May 2008 by Nortel, in which a basic concept and idea thereof is concluded according to a document of IEEE C802.16m Contribution with a serial number of IEEE C802.16m-08/347. Moreover, the other two documents are U.S patents disclosed by Texas Instruments on Dec. 11, 2008, which are respectively “Reduced Search Space Technique for Codeword Selection, US 2008/0304463 A1” and “Low Complexity Pre-coding Matrix Selection, US 2008/0304464 A1”.
These three documents have a common assumption that a wireless channel is slowly changed. Therefore, within a time period, at a certain time point t0, a full search (or exhaustive search) is performed to all of the codebooks, which is a general operation, and at the next several time points, a small range searching is performed while taking the pre-coding matrix selected at the previous time point as a reference, wherein the small range is a predetermined range. It should be noticed that the whole procedure has to be repeated every a time period T, so as to avoid generating a severe error propagation.
Assuming the codebook has L pre-coding matrices, and each of the pre-coding matrices specifies p−1 closest pre-coding matrices. The method to specify p−1 closest pre-coding matrices is to categorize the matrices according to an inner product of two pre-coding matrices, and the greater the inner product is, the higher the similarity of the two pre-coding matrices is, so that a next search is performed on p pre-coding matrices including the p−1 closet pre-coding matrices and the pre-coding matrix itself. Wherein, the categorization can be performed according to an off-line operation, and after the pre-coding matrices are categorized, a transmitter and a receiver have to store a relation table of the pre-coding matrices, which may occupy extra memories.
Within the time period T, the search can be performed on the categorized pre-coding matrices, and when each integral time T is reached, the conventional full search has to be performed. Therefore, though a required searching range of the aforementioned codebook searching method can be reduced in most of the time, the full search with the highest complexity has to be performed every a time period.